Wound film capacitor and method of winding same about its lead wires

ABSTRACT

Wound film capacitor and method of making wherein at least two layers of shrinkable thermoplastic dielectric are sandwiched between two layers of metalized or, preferably, foil, electrodes. A pair of lead wires are then positioned against the outer surfaces of the foils and rotated together in the manner of mandrels to wind the film and foil layers into a capacitor body. Flat, or otherwise deformed portions formed on a short portion of one end of each lead wire which is outside of the capacitor during winding are then drawn into the center of the capacitor by pulling on the other end of the wires. The capacitor body is then heated to shrink the dielectric and thereby cause the leads to be held in extremely firm engagement with the foil layers. Since the foils encircle at least 270* of the periphery of the lead wires, a very firm, low resistance, pressure bond is made which renders the capacitor able to withstand substantial amounts of heat applied during a soldering operation. The elimination of mandrel holes allows the capacitor to be extremely compact and usable for many applications without the addition of additional thicknesses of sealing materials.

waited States Patent Rayburn 1 Aug. 28, 1973 WOUND FILM CAPACITOR ANDMETHOD Primary Examiner-Charles W. Lanham 0F WINDING SAME ABGUT ITS LEADIrs isidn t Exiiiriinef-Carlii. Wall 7 WIRES Attorney-Robert W. Heart eta1.

[76} Inventor: Charles C. Rayburn, 8501 West ggigens Rd., Falls Church,Va. 57 AESTRACT 31 Wound film capacitor and method of making wherein[22] Flled: 7! 1971 at least two layers of shrinkable thermoplasticdielec- [21] Appl. No.: 187,570 tric are sandwiched between two layersof metalized or, preferably, foil, electrodes. A pair of lead wires areRelated Appuufim Data then positioned against the outer surfaces of thefoils [62] Division of Ser. No. 16,834, March 5, 1970, Pat No. androtated together in the manner of mandrels 10 Wind 3,638,036 the filmand foil layers into a capacitor body. Flat, or otherwise deformedportions formed on a short portion U-S.

1 of one end of each lead wire is outside of the ca- HEW. itor are thedrawn the center of Search of the capacitor an the other end of the317/260; 242/56 wires. The capacitor body is then heated to shrink thedielectric and thereby cause the leads to be held in ex- 1 References CMtremely firm engagement with the foil layers. Since the UNITED STATESPATENTS foils encircle at least 270 of the periphery of the lead3,391,313 7/1968 Hevey 317/260 x wires a very resismce Pressure is 3,233225 11 19 x m 317 2 0 X made which renders the capacitor able towithstand 3,229,174 1/1966 Marchewka 317/260 substantial amounts of heatapplied during a soldering 3,221,227 11/1965 Devaney 317/260 Xoperation. The elimination of mandrel holes allows the 3,153,180 10/1964Be11more.... 29/2142 X capacitor to be extremely compact and usable formany 3,009,086 11/1961 Rice et a1 317/260 X applications without theaddition of additional thicknesses of sealing materials.

6 Claims, 16 Drawing Figures \ageas Patented Aug. 28, 1973 WOUND FILMCAPACITOR AND METHOD OF WINDING SAME ABOUT ITS LEAD WIRES This is adivision, of application Ser. No. 16,834, filed 3-5-70 now US. Pat. No.3,638,086.

BACKGROUND Film-foil capacitors are quite widely used because theyofi'er relatively high quality at a relatively low cost. Conventionally,leads have been applied to these capacitors after they are wound, eitherby soldering them to the ends or by heating them to embed them in theplastic as described and claimed in my US. Pat. No. 3,040,415. Attemptshave been made in the past to anchor leads to foil strips, such as bysoldering, and then to wind the capacitor about the leads and foilstrips as exemplified by U.S. Pat. Nos. 2,579,462 and 3,229,174, andGerman Patent 886,634. Attempts have also been made to wind a capacitorabout a pair of wires, one of which could become one of its lead wireswith the other being withdrawn after winding, as exemplified by US. Pat.No. 2,790,607. An additional patent, US. Pat. No. 1,960,944, showswinding a capacitor about a pair of cylindrical insulating tubes withlead wires being inserted through the tubes after winding. Each of theseprior art constructions, as well as all other constructions of which Iam aware, is limited in its capability of providing one or more of thedesirable properties of: compactness, lead strength, economy andsimplicity of manufacture, an extended value and voltage range,adaptability to various lead configurations, low contact resistance, asolid core to stabilize performance characteristics and eliminate thetendency to unwind often found in mandrel-wound capacitors, and a largeinitimate contact area between the foils and the lead wires to conductand dissipate heat especially the large quantities of heat oftendeveloped as the capacitor is soldered into its circuit applicatron.

SUMMARY The various deficiencies of the prior art are overcome by thecapacitor of the present invention which consists broadly of two or morelayers of shrinkable thermoplastic dielectric film positioned, insandwiched relation, between a pair of foil layers or strips, theoutside surfaces of the foil layers being engaged by a pair of axiallyextending lead wires. The lead wires have flats or other deformationsthereon which lock them to the foils after winding and heat shinking thedielectric and prevent their being rotated in or being pulled out of thecapacitor. Since it has been found that the foil and dielectric stripstend to wrinkle when wound about a lead of non-uniform cross-section,the capacitor is wound on smooth portions of the lead wires and thedeformed lead portions are pulled axially inside the capacitor after thewinding has been finished and the wraps sealed together, but prior tothe heat shrinking operation.

My improved capacitor utilizes the heat shrinkage characteristics ofbi-axially oriented films. Such films include polyester, polypropylene,polystyrene, polycarbonate and combinations thereof. A variation inparticular performance characteristics such as the dissipation factorand the temperature-capacitance relation, may be obtained by using aparticular film or combination of films having the characteristicdesired.

The capacitor has the advantage that relatively large, rigid leads areused, e.g., No. 20 or No. 22 AWG. The leads can be formed in eitheraxial, radial or stand-up styles.

The general extent of the value-voltage range of the capacitor is fromabout pf 600 V to 0.1 mfd 200 V. although it is of course possible toextend the range beyond these values.

Since the capacitor includes no mandrel hole to occupy space and sinceit is, for most purposes, able to be used in its own thinself-encasement, it is obvious that it should provide the smallestpossible film-foil capacitor obtainable.

Although the capacitor system is designed for selfencasement, a varietyof other coating systems are usable as permitted within the style andtemperature limitations of the capacitor materials. For example, theunits may be epoxy dipped, phenolic dipped and waxed, molded, wrap andfilled, potted or canned.

Since the lead wires serve as the winding mandrel and remain in placeafter winding, there is no possibility of the capacitor graduallyunwinding into the mandrel hole as is often possible in conventionalwinding systems using a retractable mandrel. The solid core of thecapacitor produced by the lead wires is thus a basis for stableperfomiance characteristics. Since the lead wires are mounted at one endof the foil strips, the capacitor will have greater self-inductance thanthe extended foil form of construction wherein the leads are soldered orotherwise in contact with a substantial member of turns of thecapacitor.

Unlike conventional heat shrunk tab type construction, such asexemplified by US. Pat. No. 2,735,970, the lead wires have pressurecontact over the entire foil width and for approximately 270 aroundtheir peripheries. This large amount of intimate pressure contact notonly assureslow contact resistance, but also permits the foils and leadwires to conduct and dissipate heat within the capacitor section andthus protect the dielectric from overheating in localized hightemperature areas as the capacitor is soldered into its circuitapplication. By the provision of deformations such as fiatted regions onthe leads centrally of the capacitor section, the leads are stronglylocked against translation and rotation. In one lead strength testperformed on a capacitor made in accordance with this invention, a l6pound pull was necessary to pull out a lead wire made of wire having adiameter of 0.031 inches which was flattened so as to be reduced incross-section to 0.0l0 inches in the center of the capacitor. In anothercapacitor having a lead wire of the same diameter, it took 8 pounds topull out a lead wire which had been reduced in cross-sectional thicknessto 0.015 inches.

My improved capacitor utilizes at least two thicknesses of dielectricbetween the electrode foils and the lead wires. This double filmthickness principally serves to maintain the high dielectric strengthand insulation resistance in this critical mandrel region wherein filmdamage is more likely to occur than in other portions of the capacitorsection. The double films also, however, can tolerate a highertemperature during lead soldering and thus help prevent lead solderingfrom degrading the dielectric strength of the capacitor. In addition,the presence of a double film between the electrodes and the first turnof the capacitor section reduces the capacitance per unit length ofelectrode in this region. This factor permits a better yield of lowvalue capacitors by allowing the use of longer electrode foils.

BRIEF DESCRIPTION OF THE DRAWINGS FIGS. 1, 2 and 3 are endcross-sectional 'views showing, respectively, the positions of thevarious foil and dielectric layers (enlarged for clarity) of thecapacitor relative to each other and the lead wires: prior to winding;after one-half revolution; and after one full revolution of winding;

FIG. 4 is a diagrammatic end view of the capacitor section illustratingthe sealing and cutting off of the outer dielectric layer from itssupply;

FIG. 5 is a top plan view of the capacitor immediately after a windingoperation;

FIG. 6 is a top plan view similar to FIG. 5 wherein each of the leadwires has been deformed over a short section of its length;

FIG. 7 is an end sectional view of the deformed portion of one of thelead wires in FIG. 6 taken on the line FIG. 8 is a view similar to FIG.6 illustrating an intermediate position in the pulling apart of the leadwires to bring the deformed sections thereon inside the capacitor; v

FIG. 9 is a plan view similar to FIG. 8 showing the relationship of thelead wires to the capacitor after the deformed portions of the leadwires have been pulled completely into the capacitor and the dielectricheat shrunk around them;

FIG. 10 is a diagrammatic end cross-sectional view illustrating thegeneral relationship of the lead wire cross-sections and the adjacentcapacitor foil and film wraps after a winding operation and before thedeformed lead sections of FIG. 6 are pulled into the center of thecapacitor;

FIG. 11 is a diagrammatic end cross-sectional view similar to FIG. 10except that it shows the relationshp of the lead wire cross-sections tothe adjacent capacitor wraps after the deformed lead portions have beenpulled into the center of the capacitor and the dielectric film shrunkas in FIG. 9;

FIG. 12 is a view similar to FIG. 9 except that it shows a modificationwhere the lead wires are bent into a radial configuration;

FIG. 13 is a view similar to FIG. 6 except that it shows a modificationwherein the wires have been deformed at the same end of the capacitor;

FIG. 14 is a view similar to FIG. 12 showing the leads after they havebeen pulled into the capacitor;

FIG. 15 is a view similar to FIG. 14 but showing the leads after thecapacitor has been heat shrunk and the leads have been bent and cut offto form stand-up leads; and

FIG. 16 is a view similar to FIG. 1 illustrating a modification whereina pair of one-side metalized dielectric strips are used instead ofseparate strips of film and foil.

DESCRIPTION OF THE PREFERRED EMBODIMENT AND PROCESS FIGS. l-9 illustratethe process steps involved in making the improved capacitor 10 of theinvention. The winding of the capacitor is shown in FIGS. 1-4. Theinitial winding step comprises placing at least one pair ofthermoplastic dielectric film strips 12, 14 in sandwiched relationshipbetween a pair of foil strips l6, l8. Depending upon the voltage ratingthat the capacitor must have, the transverse width of the foil stripsmay be of varying amounts less than the width of the dielectric stripsto provide the desired width of insulating margin portions. A pair ofelongated lead wire members 20, 22, are placed on opposite sides of thefilm and foil layers 12, I4, l6, l8 and mounted in winding head (notshown) which will rotate them in a clockwise direction about a commoncenter between them so as to wind the layers in the manner shown inFIGS. 2 and 3 (which show the relative positions of the various elementsafter one-half and one full revolution respectively). In FIG. 3, it canbe seen that the foil layers l6, 18 will be held in intimate engagementwith their respective lead wires 20, 22 over approximately 270 of thewire peripheries. It can be further seen that the foil layers l6, 18 areseparated from each other at the center of the capacitor by a doublethickness of dielectric material. As previously noted, this doublethickness not only protects the dielectric layers against damage in thiscritical central region of the capacitor, but also increases the amountof heat which may be applied to the lead wires 20, 22 during asubsequent lead soldering operation without damaging the capacitor.

The winding of the capacitor is continued until the desired lengths ofthe respective foils 16, 18 for the capacitance desired have been woundin and protected by several outer wraps of the plain dielectric strip 12as can be seen in FIG. 4. When sufficient outer wraps of dielectric 12have been applied, the wraps may be sealed to each other in anyconventional manner such as by gluing or the application of adhesivetape or by heat sealing, utilizing a combination heat sealing andcutting bar 28 in the manner set forth in US. Pat. No. 2,950,070.

At the completion of the winding and sealing operation, the capacitorwill appear as shown in FIG. 5 with the lead wires 20, 22 extending fromeither end of the capacitor body member 26. Since the circular leadwires are relatively easily slidable relative to the capacitor body 26,a deformation such as a flattened region 20b, 22b, as seen in FIGS. 6and 7, is formed on each lead wire 20, 22 by means of a relatively largediameter (about 1 inch) cylinder (not shown) which presses the lead wireagainst a flat plate (not shown). After the lead wires have beendeformed, from a thickness d to a thickness d which is preferablyone-third to one-half the thickness d, they are pulled axially relativeto the capacitor body 26 (FIG. 8) so as to bring the deformed portions20b, 22b into contact with the foil layers l6, 18 at the center of thecapacitor body. Since the deformation formed by a cylinder in contactwith a circular wire member provides a smooth transition section betweenthe two cross-sections of the wire, the deformed sections 20b, 22b willreadily slip over the surfaces of the foils 16, 18 without injuringthem. Although the type of foil used is not extremely critical, it hasbeen found that tin foil provides somewhat better results than aluminumfoil for the reason that tin foil will accommodate the heat shrinking ofthe dielectric without the puckering which is sometimes produced inaluminum foil. Furthermore, tin foil is very malleable and allows thelead wires to slip quite freely over its surface.

After the deformed portions 20b, 22b of the lead wires have been pulledinto the center of the capacitor body 26, the capacitor is subjected toheat for a limited period of time in order to cause the dielectric filmlayers l2, 14 to shrink and thereby exclude air from between the layersand bring the foil layers l6, 18 into firm intimate pressure engagementwith the lead wires 20, 22. Although the capacitor, after heatshrinking, (FIG. 9) is ready for use without additional sealing, it isof course possible to apply additional sealing materials if desired orto bend the leads 20, 22 in any manner desired such as into the radiallead style shown in FIG. 12.

The nature of the locking engagement between the lead wires 20, 22 andthe most nearly adjacent film and foil layers, indicated collectively at32, can be seen in FIGS. 10 and 11. FIG. 10, which is a sectional viewtaken along the line 10-10 in FIG. 6, shows how the layers 32 of thecapacitor conform to the lead wire surfaces for about 270 of theperiphery of the lead wires 20, 22 when the lead wire cross-section isround. In FIG. 11, it can be seen that the degree of peripheral contactof the heat shrunk layers 32 with the flattened lead wires is evengreater than 270. Since the length of the outer periphery of theflattened or otherwise deformed lead wire cross-sections b, 226 aresubstantially the same or only slightly greater than that of thecircular lead wires 20, 22, the foils l6, 18 will not be damaged as thedeformed portions 20b, 22b of the lead wires are slide into the centerof the capacitor. A comparison in FIG. 11 of the initial lead wirecross-section shown in dotted lines with the final cross-section shownin solid lines, indicates that the thickness of the final capacitor willbe reduced from its thickness after winding. It can also be seen thatthe final flattened shape of the lead wires causes them to fill upalmost the entire internal volume of the capacitor and thus renders thecapacitor more stable.

FIGS. 13-15 illustrate a modified form of capacitor having stand-up,rather than axial, leads. This capacitor style is obtained by merelyforming the flatted portions 22a, 22b on the lead wires at the same endso that when they are pulled into the capacitor body 26', their oppositeends 20', 22 will project in the same direction. After the capacitorbody 26' in FIG. 14 is heated to shrink the dielectric, it will beassume the shape shown in FIG. I5 at which point the lead wires 20', 22'may be bent in diverging directions and cut off as seen in FIG. 15.

Although the slipped lead concept of the invention set forth hereinfinds its greatest utility as applied to film foil capacitors, it wouldalso be of some usefulness as applied to capacitors made of two stripsof one-side metalized dielectric material. Such strips, shown in FIG. 16at 112, 114 as having metalizcd coatings H6, 118 in contact with leadwires 120, 122, would be wound similarly to the film and foil stripspreviously described. However, since the metalized coatings I16, 118 areof extreme thinness, the lead wires used therewith would have to beextremely smooth to avoid causing damage to them as they were slid overthe coatings.

I claim:

1. A method of making a wound capacitor comprising the steps of: placingat least a pair of thermoplastic dielectric strips and a pair ofmetallic electrode layers between a pair of lead wire members so thatthe at least two layers of dielectric are sandwiched between the pair ofelectrode layers with said electrode layers each contacting one of saidpair of lead wire members, said lead members being axially elongated andpositioned transversely of said dielectric strips; rotating said leadwire members together as a unit to wind said thermoplastic dielectricstrips and said electrode layers into a capacitor body; providing thecapacitor body with outer wraps of plain dielectric; sealing the outsidewraps of the capacitor to each other; sliding said lead wire membersaxially relative to said capacitor and moving a deformed portion of eachof said lead wire members from a position outside said capacitor body toa position inside said capacitor body; and heating said capacitor toshrink said thermoplastic dielectric and cause it to force saidelectrode layers into firm engagement with said lead wires.

2. The method of claim 1 wherein said pair of metallic electrode layerscomprise strips of metal foil.

3. The method of claim I wherein each of said electrode layers is woundin firm intimate pressure contact across its entire width with at least270 of the periphery of its respective lead wire members.

4. The method of claim 3 wherein said electrode layers comprise stripsof metal foil.

5. The method of claim 4 wherein said metal foil is tin foil.

6. The method of claim 1 wherein the pair of dielectric strips areplaced between the lead wire members so that terminal ends of the stripsof dielectric overlap in the longitudinal direction of said strips.

1. A method of making a wound capacitor comprising the steps of: placingat least a pair of thermoplastic dielectric strips and a pair ofmetallic electrode layers between a pair of lead wire members so thatthe at least two layers of dielectric are sandwiched between the pair ofelectrode layers with said electrode layers each contacting one of saidpair of lead wire members, said lead members being axially elongated andpositioned transversely of said dielectric strips; rotating said leadwire members together as a unit to wind said thermoplastic dielectricstrips and said electrode layers into a capacitor body; providing thecapacitor body with outer wraps of plain dielectric; sealing the outsidewraps of the capacitor to each other; sliding said lead wire membersaxially relative to said capacitor and moving a deformed portion of eachof said lead wire members from a position outside said capacitor body toa position inside said capacitor body; and heating said capacitor toshrink said thermoplastic dielectric and cause it to force saidelectrode layers into firm engagement with said lead wires.
 2. Themethod of claim 1 wherein said pair of metallic electrode layerscomprise strips of metal foil.
 3. The method of claim 1 wherein each ofsaid electrode layers is wound in firm intimate pressure contact acrossits entire width with at least 270* of the peripherY of its respectivelead wire members.
 4. The method of claim 3 wherein said electrodelayers comprise strips of metal foil.
 5. The method of claim 4 whereinsaid metal foil is tin foil.
 6. The method of claim 1 wherein the pairof dielectric strips are placed between the lead wire members so thatterminal ends of the strips of dielectric overlap in the longitudinaldirection of said strips.